1. Field of the Invention
The present invention relates to a torque transmitting device for transmitting torque by utilizing the viscosity of a fluid.
2. Description of the Relevant Art
Viscous shear couplings are known as a device for transmitting torque by utilizing the viscosity of a fluid. The viscous shear couplings generally comprise a casing having a cylindrical barrel, a shaft rotatably supported in the casing in concentric relation to the barrel, a fluid chamber defined between the inner surface of the casing and the peripheral surface of the shaft, a plurality of outer plates rotatable in the fluid chamber in unison with the casing, a plurality of inner plates disposed in the fluid chamber alternately with the outer plates and rotatable with the shaft, and a highly viscous fluid filled in the fluid chamber.
The viscous shear coupling is widely used for connecting two coaxial shafts to each other and in differentials and the like. Japanese Laid-Open Patent Publication No. 58-50349 (British Patent Application No. 8119769) discloses such a viscous shear coupling incorporated in an automotive differential.
When transmitting torque with a viscous shear coupling, the temperature of the fluid rises since torque transmission relies upon the viscous resistance developed when the outer and inner plates shear the fluid present therebetween. If the fluid flows in the fluid chamber by circulation or convection, localized temperature increases of the fluid are prevented and hence the fluid has increased durability for maintaining desired torque transmission characteristics. With the conventional coupling structure described above, however, the outer peripheries of the outer plates are fitted in the inner peripheral surface of the casing, and the alternate outer and inner plates are closely positioned. For the fluid to flow in the fluid chamber, the fluid has to pass through a narrow labyrinth defined by the outer plates, the inner plates, and the casing. Therefore, the fluid cannot smoothly circulate by convection, with the result that an unwanted localized temperature rise of the fluid cannot be prevented effectively.
Since the flow path in the fluid chamber is labyrinths, the fluid cannot quickly be supplied into and discharged from the fluid chamber during assembling or servicing the fluid chamber. Moreover, inasmuch as the fluid chamber is filled with air before being filled with fluid a small amount of air may be trapped therein, during filling which, cannot quickly be purged.
Where the viscous shear coupling is combined with an automotive differential as disclosed in British Patent No. 8119769, if only one of the lateral axles rotates differentially with respect to the casing of the coupling, then only the temperature of fluid present near that axle may rise. This may develop different torque transmission characteristics for the axles during usage over a long period of time.